Most introduced species establish no population at all. A small proportion establish, and a smaller proportion cause significant harm, which makes prediction difficult and prevention important.
The escape from constraints
In its native range, a species is held in check by predators, parasites, diseases and competitors that evolved alongside it.
Arriving without them, it can reach densities impossible at home.
Which is the enemy release hypothesis, and it explains a substantial share of successful invasions.
It also explains why biological control — introducing a specific natural enemy — sometimes works and why it has sometimes caused further problems.
Naive prey
Native species that have not encountered a predator type may lack appropriate responses.
Which is why island species have been so severely affected by introduced predators, having evolved without mammalian predators entirely.
A substantial proportion of documented bird extinctions have occurred on islands, with introduced predators the principal cause.
Ecosystem engineering
Some invaders change the physical environment rather than simply competing.
Plants altering fire regimes, soil chemistry or hydrology.
Animals altering vegetation structure through grazing or through burrowing.
Which affects everything else in the system, and it is why these invasions are hardest to reverse.
Disease
Introduced pathogens have caused some of the most dramatic effects.
A fungal disease has driven amphibian declines and extinctions across multiple continents.
Tree diseases have removed formerly dominant species from entire forests.
Which spread through trade in plants, timber and animals, and it is why biosecurity focuses heavily on these pathways.
The lag
Invasions frequently show a delay between establishment and rapid spread.
Which means a species present at low density for years can expand suddenly, and the period of apparent harmlessness is misleading.
The explanations include population growth from a small founding number, adaptation to local conditions, and reaching a threshold of suitable habitat.
It also means the current inventory of established non-native species includes future problems that have not yet manifested.
Pathways
Deliberate introduction for agriculture, ornament, sport and pest control.
Escape from captivity or cultivation.
Transport as contaminants of traded goods.
Ballast water and hull fouling in shipping, which moves marine organisms globally.
Which is why prevention concentrates on pathways rather than on individual species, since the number of potential invaders is unmanageable.
Eradication and control
Eradication is achievable on islands and in confined systems, with a substantial record of success particularly for mammals.
It requires removing every individual, which is why partial success can be worse than none if the population rebounds.
On continents, control rather than eradication is generally the realistic objective, at ongoing cost.
Early detection and rapid response, before establishment, is dramatically cheaper than anything afterwards, and it is where investment is most effective.
The complication
Not all non-native species cause harm, and some provide benefits.
Which means blanket hostility to non-native species is neither accurate nor useful.
The relevant question is impact rather than origin, and assessing it requires evidence rather than assumption.
Economic cost
Estimates of the global economic cost of invasive species run to very large figures, covering agricultural losses, infrastructure damage and management expenditure.
Which has driven policy attention more effectively than the ecological argument, and the estimates carry substantial uncertainty.
The cost of prevention is consistently found to be far lower than the cost of management after establishment.
Biosecurity in practice
Border inspection, treatment requirements for imported goods, ballast water regulation and restrictions on movement of soil and plant material.
Which are the practical measures, and they operate against enormous volumes of trade.
Risk assessment targets inspection at high-risk pathways rather than attempting comprehensive coverage.
What individuals can do
Not releasing pets or aquarium contents.
Cleaning boots, boats and equipment between sites, particularly between water bodies.
Not moving firewood between regions, which transports wood-boring insects.
And checking before planting, since several widely sold ornamental plants are invasive in some regions.
Island eradications
The clearest conservation successes in this area.
Hundreds of islands have had invasive mammals removed, with documented recovery of seabird colonies, reptiles and plants.
Which requires complete removal, biosecurity to prevent reinvasion, and generally considerable expense.
Techniques have improved substantially, and the size of island that can be treated has increased over time.
Novel technologies
Genetic approaches, including gene drives, have been proposed for invasive control.
Which raises substantial governance questions, since a self-spreading genetic modification could not be recalled and would not respect borders.
International discussion is ongoing and no field release has occurred for vertebrates.
Prediction
Risk assessment frameworks estimate the likelihood a species will establish and cause harm.
Which uses climate matching, traits associated with invasiveness, and history of invasion elsewhere.
Previous invasion elsewhere is the strongest single predictor, which makes international information sharing genuinely valuable.
Which is why horizon scanning exercises, identifying species likely to arrive before they do, have become standard practice in several countries.